Recently, economic growth and changes in lifestyle are leading to changes in eating habits. The main causes of rising overweight and obesity rates in contemporary people are consumption of high-calorie foods such as fast foods and lack of exercise. World Health Organization (WHO) estimates that more than 1 billion people worldwide are overweight and at least 300 million of them are clinically obese. In particular, 250,000 people die each year in Europe and more than 2.5 million people worldwide die each year as a result of being overweight (World Health Organization, Global Strategy on Diet, Physical Activity and Health, 2004).
Being overweight and obese increases blood pressure and cholesterol levels to cause occurrence or exacerbation of various diseases such as cardiovascular disease, diabetes, and arthritis, and are also main causes of rising incidence rates of arteriosclerosis, hypertension, hyperlipidemia or cardiovascular disease in children or adolescents as well as in adults.
Obesity is a severe condition that causes various diseases worldwide. It is thought to be overcome by individual efforts, and it is also believed that obese patients lack self-control. However, it is difficult to treat obesity, because obesity is a complex disorder involving appetite regulation and energy metabolism. For the treatment of obesity, abnormal actions associated with appetite regulation and energy metabolism should be treated together with efforts of obese patients. Many attempts have been made to develop drugs capable of treating the abnormal actions. As the result of these efforts, drugs such as Rimonabant (Sanofi-Aventis), Sibutramin (Abbott), Contrave (Takeda), and Orlistat (Roche) have been developed, but they have the disadvantages of serious adverse effects or very weak anti-obesity effects. For example, it was reported that Rimonabant (Sanofi-Aventis) shows a side-effect of central nerve disorder, Sibutramine (Abbott) and Contrave (Takeda) show cardiovascular side-effects, and Orlistat (Roche) shows only 4 kg of weight loss when taken for 1 year. Unfortunately, there are no therapeutic agents for obesity which can be safely prescribed for obese patients.
Many studies have been made to develop therapeutic agents for obesity which do not have the problems of the conventional anti-obesity drugs. Recently, glucagon derivatives have received much attention. Glucagon is produced by the pancreas when the level of glucose in the blood drops resulting from other medications or diseases, hormone or enzyme deficiencies. Glucagon stimulates glycogen breakdown in the liver, and facilitates glucose release to raise blood glucose levels to a normal range. In addition to the effect of increasing the blood glucose level, glucagon suppresses appetite and activates hormone-sensitive lipase (HSL) of adipocytes to facilitate lipolysis, thereby showing anti-obesity effects. One of the glucagon derivatives, glucagon like peptide-1 (GLP-1) is under development as a therapeutic agent for hyperglycemia in patients with diabetes, and it functions to stimulate insulin synthesis and secretion, to inhibit glucagon secretion, to slow gastric emptying, to increase glucose utilization, and to inhibit food intake. Exendin-4 is isolated from lizard venom that shares approximately 50% amino acid homology with GLP-1 and is also reported to activate the GLP-1 receptor, thereby ameliorating hyperglycemia in patients with diabetes. However, anti-obesity drugs including GLP-1 are reported to show side-effects such as vomiting and nausea.
As an alternative to GLP-1, therefore, much attention has been focused on oxyntomodulin, a peptide derived from a glucagon precursor, pre-glucagon that binds to the receptors of two peptides, GLP-1 and glucagon. Oxyntomodulin represents a potent anti-obesity therapy, because it inhibits food intake like GLP-1, promotes satiety, and has a lipolytic activity like glucagon.
Based on the dual function of the oxyntomodulin peptide, it has been actively studied as a drug for the treatment of obesity. For example, Korean Patent No. 925017 discloses a pharmaceutical composition including oxyntomodulin as an active ingredient for the treatment of overweight human, which is administered via an oral, parenteral, mucosal, rectal, subcutaneous, or transdermal route. However, it has been reported that this anti-obesity drug including oxyntomodulin has a short in vivo half-life and weak therapeutic efficacy, even though administered at a high dose three times a day. Thus, many efforts have been made to improve the in vivo half-life or therapeutic effect of oxyntomodulin on obesity by its modification.
For example, a dual agonist oxyntomodulin (Merck) is prepared by substituting L-serine with D-serine at position 2 of oxyntomodulin to increase a resistance to dipeptidyl peptidase-IV (DPP-IV) and by attaching a cholesterol moiety at the C-terminal to increase the blood half-life at the same time. ZP2929 (Zealand) is prepared by substituting L-serine with D-serine at position 2 to enhance resistance to DPP-IV, substituting arginine with alanine at position 17 to enhance resistance to protease, substituting methionine with lysine at position 27 to enhance oxidative stability, and substituting glutamine with aspartic acid and alanine at positions 20 and 24 and asparagine with serine at position 28 to enhance deamidation stability. However, even though the half-life of the dual agonist oxyntomodulin (Merck) was enhanced to show half-life 8-12 minutes longer than the native oxyntomodulin, it still has a very short in vivo half-life of 1.7 hr and its administration dose is also as high as several mg/kg. Unfortunately, oxyntomodulin or derivatives thereof have disadvantages of daily administration of high dose due to the short half-life and low efficacy.